In the treatment of patients suffering from respiratory ailments, such as emphysema where the patient's lung capacity is severely restricted, it is common practice to provide the patient with a source of oxygen-enriched gas. Typically, this source of oxygen-enriched gas is provided from a pressurized oxygen cylinder which may be located remotely from the patient in a hospital and supplied through suitable tubing (central storage type) or may be an individual cylinder located at the patient's bedside. Since many of these ailments are chronic and require extended therapy, portable oxygen cylinders which the patient may use at home have been developed.
While the use of individual cylinders provides the necessary life-sustaining therapy for these patients, the cylinders themselves present several problems when used in the home. Specifically, since these cylinders contain enriched-oxygen gas, they present a constant danger of fire and explosion during use. The individual cylinders have limited capacity, and therefore must be serviced and replaced routinely, thereby increasing the cost of therapy. In addition, there may also be leakage problems which may unexpectedly diminish the capacity of a cylinder so that the patient is left with inadequate therapy gas.
Atmospheric air, which contains about 20% oxygen and 78% nitrogen, provides a vast and abundant source of oxygen. However, until recently, technology for extracting oxygen economically for individual use has been lacking. With the development of thin permselective membranes, such as those of plastics, such as silicone rubber, polyphenylene ethers, and the like, and associated systems technology, feasible separation of gases has been achieved.
The separation of gases in such membrane systems technology is based on the selective permeability of certain materials. The term "selective permeability" means that one gas in a mixture will permeate through a membrane faster than a second gas, but this is not to suggest that one gas passes through the membrane to the complete exclusion of all others. Rather, a difference in the flow rate of two molecular species through a permeable membrane results so that the gas mixture on one side of the membrane is depleted in concentration of the more permeable component and the gas on the opposite side of the membrane is enriched with the more permeable component.
In either case, because of the nature of the system, nitrogen dioxide (NO.sub.2), which is in equilibrium with dinitrogen tetroxide (N.sub.2 O.sub.4), has a two-fold detrimental effect on membrane oxygen enrichers: (i) the membrane array enriches the NO.sub.2 in permeate output because NO.sub.2 has a very high rate of permeation through membranes, as compared to oxygen and nitrogen (more than 12.times. higher with a dimethyl silicone membrane, for example); and (ii) most membrane materials are chemically attacked and ultimately destroyed by NO.sub.2. Nitrogen dioxide is contraindicated in therapeutic gas streams because it is poisonous, and it reacts with body fluids to form acids. For these reasons, it is important to remove NO.sub.2 from such membrane enrichment processes and techniques for this are known, but are not entirely satisfactory. For example, the ambient air feed can be wet-scrubbed with caustic, but this unduly entrains moisture. The ambient air can be passed over a molecular sieve, but this is not rapid and efficient. Finally, the air can be passed over soda lime, but this is rapidly exhausted, generates heat and is generally inefficient.
It has now been discovered that feeding the ambient air into contact with triethanolamine efficiently and rapidly removes nitrogen dioxide to only a small fraction of that which is originally present. Then the so-treated air, depleted in nitrogen dioxide, can be delivered to the membrane cell array without fear of having this pollutant increased to therapeutically dangerous levels, and with minimization of membrane deterioration due to adverse chemical reactions.